Doesn’t really matter, those chargers can take in from about 90V to about 250, 260V. Forget what the sticker says about 110~230, they’re designed for unstable voltage operation (not actually designed for that, but they don’t wanna make different ones for EU and UK/US voltages coz it costs more, plus people bitch about not being able to use them abroad, so they just make the same ones and ship with different plugs). Trust me, they can handle voltage swings pretty darn good. They’re SMPS power supplies, they’re designed to output the same voltage in a very wide range of input voltages.
If you really think that a battery will help (it might help… in some cases… depending on how it’s built), just leave the battery on even if it’s dead. It might work as voltage dumper in some cases, but as I said, it depends on the design.